"Re-entry is expected sometime during the afternoon of Sept. 23, Eastern Daylight Time," said NASA in an update this morning. "The satellite will not be passing over North America during that time period."

The Aerospace Corporation, a private firm that is tracking UARS, offered a more specific prediction, saying the satellite would likely come down off the coast of Chile at 6:06 p.m. EST. But William Ailor, who heads the company's center for orbital and re-entry debris studies, said the time and location would almost undoubtedly change as Friday afternoon approaches.

NASA repeated that the risk to people or property is "extremely small."

As of this morning, UARS had an altitude of about 115 miles, skimming the uppermost reaches of the atmosphere at more than 17,000 mph. At some point, a little like a stone skipping over a pond, it will encounter enough resistance that it will no longer be able to keep moving at orbital speeds.

This is the largest NASA satellite to fall back to Earth uncontrolled since Skylab in 1979. UARS is 35 feet long and weighs about 6 tons, but most of it will burn up on re-entry. The little that doesn't is what worries NASA.

Nicholas Johnson, NASA's chief orbital debris scientist, said their analysis shows that 26 parts of UARS are dense enough to make it to the Earth's surface.

"These 26 components, which we anticipate will survive all the way down, will be going at a moderate velocity of tens to hundreds of miles an hour," he said. "All these 26 have been identified as potentially causing damage if they hit a structure or a person, but the odds of that are very, very, low."

Johnson said any one person's chances of getting hit by debris are tiny -- something like 1 in 21 trillion. The chances that of the 7 billion people on Earth, one of them, somewhere, could be hit are more like 1 in 3,200.

About 150 tons worth of meteorites, in pieces large and small, crash to Earth every day. Some are visible, such as the meteor spotted over California last week, but most of the debris is so small it's never found.

What makes UARS so remarkable is its size and the advance warning from NASA officials that it's coming. Because of the varying density of the upper atmosphere, it will be hard to predict closely when it will plunge back to Earth until about two hours before it actually does.

"The sun is constantly changing the output of energy, which affects the atmosphere, and that can accelerate of delay the re-entry of the satellite," Johnson said.

UARS -- short for Upper Atmosphere Research Satellite -- was carried in the payload bay of the space shuttle Discovery in 1991. It was launched into an orbit that took it 57 degrees north and south of the equator.

That "pretty much encompasses the entire populated world," said Johnson. Its mission was to monitor the atmosphere from above, seeing how the environment was changing over time.

UARS originally orbited the planet at an altitude of 350 miles, but even up there, a minute amount of air was enough to slow it over the years.